Recently, magneto-optical memory devices such as magneto-optical disks, have been viewed with interest as rewritable high density and large capacity memory devices. Especially, a demand for a magneto-optical memory device, whereon a so-called overwriting operation can be carried out without erasing the previously written information, is increasing.
For the overwriting method, the magnetic field modulation and the light intensity modulation which utilizes an exchange coupled film have been proposed.
An overwriting operation through the magnetic field modulation is carried out in the following way. While projecting a light beam of a constant intensity on a predetermined recording area of the magneto-optical memory device, a magnetic field, whose direction reverses in response to information to be recorded, is applied from a magnetic head, thereby recording information. For the magnetic head, a floating-type magnetic head provided with a slider, which is adopted in, a hard disk device, etc., is used in order to generate a strong magnetic field of high frequency.
As described in the summary of the Spring Meeting by the Applied Physics Society in 1987, page 721 (lecture number 28p-ZL-3), for the overwriting operation through the light intensity modulation, a magneto-optical memory device, having an exchange coupled film of a double layer structure consisting of a memory layer and a writing layer made of a magnetic substance laminated on a base, is employed. When carrying out the overwriting operation, first the magnetization direction of the writing layer is arranged in one direction by an initialization magnet. Then, while applying a magnetic field, whose direction is opposite to the arranged direction, to a predetermined recording area of the memory layer from a subsidiary magnet, a light beam, whose intensity varies in response to information to be recorded, is projected on the recording area, thereby recording information.
In the summary of the 13th lecture by the Japan Applied Magnetic Society in 1989, page 192 (lecture number 23aC-4), a magneto-optical memory device having an exchange coupled film of a four layer structure consisting of an initialization layer, a control layer, a subsidiary recording layer and an information recording layer laminated on a glass base was employed. Here, the magnetization direction of the initialization layer is arranged in one direction beforehand and is not reversed throughout the operation. When carrying out an overwriting operation, while applying a magnetic field, whose direction is opposite to the arranged direction, to the predetermined recording area of the information recording layer from the subsidiary magnet, a light beam, whose intensity varies in response to the information to be recorded, is projected on the recording area, thereby recording information.
However, the above overwriting operation through the magnetic field modulation creates the following problems. When a magnetic head starts floating, a slider, whereon the magnetic head is provided, slides on the magneto-optical memory device. Therefore, scratches may occur on the magneto-optical memory device, or the magnetic head may be crushed. This means that recording and reproducing of information cannot be carried out without sacrificing the non-contact operation which is the essential characteristic of the magneto-optical memory device.
Whereas, in the case of the overwriting operation through the light intensity modulation using the magneto-optical memory device mainly consisting of the exchange coupled film of a double layer structure, the following problem arises. That is, a large initialization magnet which generates a magnetic field of several kOe is required.
On the other hand, when the magneto-optical memory device mainly consisting of the exchange coupled film of a four layer structure is employed for the overwriting operation through the light intensity modulation, if the initialization layer is initialized beforehand, an additional initialization is not required. However, there is a difficulty in manufacturing the magneto-optical memory device since each layer needs to be set for a predetermined coercive force and Curie temperature.